JPS6140360A - Electrically conductive resin composition, and current limiting element using said composition - Google Patents

Abstract

PURPOSE:To obtain an electrically conductive resin composition having remarkable PTC characteristics and suitable for a current limiting element, by compounding a thermoplastic resin with a carbon black having a specific range of surface area and a graphite having an average particle diameter smaller than a specific level. CONSTITUTION:The objective composition can be prepared by mixing (A) a thermoplastic resin, practically preferably a polyetheylene, an ethylene-vinyl acetate copolymer, etc. with (B) carbon black having a specific surface area of 20-100m<2>/g and (C) graphite having an average particle diameter of <=10mum. The sum of the components B and C is 40-100pts.(wt.) per 100pts. of the component A, and the graphite ratio C/(B+C) is 0.1-0.6. The composition is kneaded homogeneously with a mixing roll, and formed in the form of a sheet. The objective current limiting element having remarkable PTC characteristics can be manufactured by attaching two or more mutually separated electrodes on the surface of the sheet.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a conductive resin composition having a positive temperature coefficient of resistance (so-called "PTC characteristics") and a current limiting element that utilizes the PTC characteristics of the composition. Regarding.

(Prior Art) Conventionally, current fuses that utilize the fusing effect caused by overcurrent have been frequently used to protect electric circuits, but these fuses have the disadvantage that they cannot be used again.

In addition, in recent years, magnetic PT whose main component is barium titanate has been developed.
C thermistors have become commercially available, but this magnetic thermistor has a high initial resistance value (usually the resistance value at 20°C is higher than 10,000 Ω-m) and cannot be made small, so there is a need for miniaturization. It is unsuitable for protection of electronic circuits intended for

On the other hand, Japanese Patent Laid-Open No. 156-161464 discloses a composition in which metal particles such as nickel, tungsten, and molybdenum and carbon plaque are blended with an S-flexible resin. The thermistor obtained with this composition has a large resistance increase in the practical temperature range, and when used as a current limiting element in electric or electronic circuits, the resistance value tends to fluctuate due to ambient temperature or self-heating, so There was a problem with the design.

(Problem to be Solved by the Invention) A current limiting element for protecting an electric/electronic circuit is connected in series with the protected circuit, and in a normal state, a current (1) corresponding to the resistance of the protected circuit flows through the element. The current also flows, and Geel heat of I''H is generated in the element (resistance value R).

The temperature of the element is maintained at a constant temperature as environmental conditions such as ambient temperature, heat dissipation conditions, and this Geel heat are in equilibrium.

-1. Since the element has a positive resistance-temperature characteristic, the specific resistance value at equilibrium temperature increases compared to the initial resistance value, but the resistance value at equilibrium temperature is equal to the initial resistance value. In comparison, if it becomes too large, the current value to the protected circuit decreases, often resulting in a current-limited state in normal conditions.

On the other hand, if the initial resistance value is lowered, the above-mentioned drawbacks are eliminated, but there is a range in which the current value of the protected circuit (ie, the current of the element) is not limited. However, if an abnormal current flows due to a short circuit, etc., the current limiting element uses Zeel heat, and the amount of heat generated depends on the resistance value, so a composition with resistance-temperature characteristics suitable for the circuit to be protected is required. Met. However, the actual resistance value of the circuit to be protected and the voltage acting on the current limiting element vary, and there is a long-awaited development of a current limiting element that can adapt to such various conditions.

(Means for Solving the Problems) In view of the above-mentioned current situation, the inventors of the present invention have conducted intensive studies and found that when mixing a specific amount of conductive particles into a thermoplastic resin, the conductive particles have a surface area within a specific range. By using carbon black and graphite having an average particle diameter below a specific value, and by setting the proportion of graphite in the total amount of conductive particles within a specific range, the initial resistance value is low and at a specified temperature. It was discovered that the resistance value can be rapidly increased, and this led to the completion of the present invention.

That is, in the present invention, 40 to 100 parts by weight of conductive particles are blended to 100 parts by weight of thermoplastic resin, and the conductive particles have a surface Wt of 20 to 100 m'', # carbon black, and an average particle diameter of 10 μm or less. The present invention relates to a conductive resin composition characterized in that the present invention is graphite, and the graphite has an abundance ratio of 0.1 to 0.6 in the total amount of conductive particles.

Another aspect of the present invention relates to a PTC current limiting element in which two or more electrodes separated from each other are provided on the surface of a molded product made of the above-mentioned conductive resin composition.

The current limiting element according to the present invention can be produced by uniformly kneading a predetermined amount of thermoplastic resin, carbon black, and graphite using a mixing roll, forming the mixture into a predetermined shape such as a sheet, film, or plate. This can be achieved by, for example, providing two or more electrodes separated from each other on the surface of the molded product.

This current limiting element has a low initial resistance value, that is, a specific resistance value (R) at 20°C of 1 to 10,000 Ω, and a low R(TP-20)/R of 1 to 10, and RTp
/R.

is at least 1000. Note that TP is the maximum resistance value of the current limiting element, that is, the peak resistance value (RTP
), R(TP-2°) is the temperature TP
The specific resistance values at a temperature 20° C. lower than (° C.) are shown.

A current limiting element having PTC characteristics as in the present invention is ideal, but in reality, the resistance usually starts to rise at a temperature lower than the TP. In determining the quality of the PTC current limiting element, the ratio of the resistance value at a temperature (T, ) lower than TP and R, that is, the resistance temperature characteristic, from 20°C to (TP
This is the most commonly used means of indicating the resistance increase slope in the temperature range of -T, ) and the ratio of R, to the resistance value (RTP) at TPO temperature, that is, the resistance change multiple. Here L is 10
If the temperature is below 30°C, the resistance will realistically start to rise steeply, making it inconvenient to use as a basis for pass/fail judgment.If T is 30°C or higher, the resistance value will rise sharply at TP. As a guidepost, L was set at 20°C because the difference from TP was too large by 1>.

Therefore, R(TP-20)/tortoise is 20°C~(TP-2
0) °C, and RTP/R# is 2
It shows the ratio of the resistance value at 0° C. and the resistance value at TP, that is, the multiple of change in resistance.

If the value of R(TP-20)/I- is less than 1, the resistance value will become smaller compared to R9 due to temperature rise, and it will show a negative temperature coefficient of resistance, and when a voltage is applied to the PTC element. ,#
As the current increases, the resistance value decreases and the current increases, which is a very inconvenient result. Furthermore, if it exceeds 10, the resistance value will change significantly due to temperature changes of the element during practical use, making it extremely difficult to design an electric circuit using the element. In some cases, a holdup device may be required to keep the element temperature constant.

Also, it is better for RTP/R4o to be as large as possible, but 1
If it is less than 000, when a practical special current is applied, the temperature range will rise due to self-heating and the allowable temperature of the element will be exceeded.Furthermore, the element itself uses the temperature change of the resistance, so the resistance change multiple The smaller one is the longer fi, 100^(
When increasing by 1, the electrode becomes larger.

Next, an example of the current limiting element according to the present invention will be explained with reference to the drawings. Figures 1 to 4 show examples of current limiting elements, and in Figure 1, the entire surface and back surface of a sheet-like molded product 1 (current limiting element main body) made of a conductive resin composition is covered. A metal foil electrode 2.3, such as a copper foil, is provided by means of heat fusion, adhesion with a conductive adhesive, or the like.

2 and 3 show other examples, in which strip-shaped electrodes 2.3 are provided along both sides of the surface of the molded product 1, and in FIG. Strip-shaped electrodes 2.3 are provided on each of the front and back surfaces.

FIG. 4 shows yet another example, in which band-shaped electrodes 2.3 are provided along both sides of the front surface of the molded product 1, and an electrode 4 is provided on the back surface to cover the entire surface thereof.

In the present invention, various thermoplastic resins can be used without particular limitation, but polyethylene, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, etc. are practically preferred. This thermoplastic resin can be crosslinked by electron beam irradiation or the like.

This thermoplastic resin contains carbon black and graphite as conductive particles in a total amount of 40 to 100 parts by weight based on 100 parts by weight of the resin. If the amount of conductive particles blended is too small, the initial resistance value will not be high, and if the blended amount is too large, when the resin composition is molded into a predetermined shape such as a sheet, the mechanical strength of the molded product will be reduced. Therefore, neither is preferable.

In the present invention, both carbon black and graphite are used together as conductive particles as described above, and the abundance ratio of graphite in the total amount of conductive particles is 0.1 to 0.6. The abundance ratio of graphite is 0
．． If less than 1, R.

and R(TP-2°)/R3 are large, and if the abundance ratio exceeds 0°6, RTP/R becomes small, which is not preferable. Note that the abundance ratio of graphite is calculated using the following formula.

Further, the carbon black used in the present invention has a surface area of 20 to 100 m/P (measured value by BET method), and the average particle diameter of graphite is 10 μm or less.

When the surface area of carbon black is as small as 20flI/f, the capacity of the carbon black becomes bulky, which makes it difficult to mix uniformly with resin and graphite, and even when this mixture is molded. A molded product having practical mechanical strength cannot be obtained, and if it exceeds 100 ne/l, the RTP/R becomes small, so neither is preferable.

Moreover, if the average particle diameter of graphite is larger than 10 μm, R(TP-20)/R2o becomes large, which is not preferable.

(Examples) Hereinafter, the present invention will be explained in more detail using examples. In addition, [parts] in the examples are "parts by weight".

Example For 100 parts of high-density polyethylene pellets (manufactured by Mikata Petrochemical Co., Ltd., trade name H2-5300B), i-side m45d/
40 parts of the carbon black of No. f and 30 parts of graphite having an average particle size of 4 μm are kneaded for 30 minutes using a mixing roll at a temperature of 165° C. (melting the polyethylene and uniformly mixing the polyethylene).

After that, cool to room temperature while maintaining the pressure at 51 w/d,
A sheet-like molded product with a thickness of 0.5 mm and a length and width of 20 c+y+ is obtained.

After crosslinking the polyethylene by irradiating this sheet-like material with an electron beam of 10 Mrad, a thickness of 0.5 sm +
.

Copper foils of 20 tM in length and width are arranged, and the temperature is 165
℃ and a pressure of 5 kg/d, and then cooled.
A current limiting element (sample number 1) having the same structure as in FIG. 1 was obtained.

Furthermore, the blending amount of carbon black and graphite,
Current limiting elements of sample numbers 2 to 7 were obtained by carrying out the same operations as for sample number 1 except that the surface area of carbon black and the average particle diameter of graphite were set as shown in Table 1.

The following carbon black and graphite were used.

(Carbon black) Average particle size 4μm...manufactured by Toyo Carbon Co., Ltd., product name A
Addressed to T 30 Average particle size 25 μm...manufactured by Toyo Carbon Co., Ltd., product name
AT Solo Average particle size 45 μm...manufactured by Toyo Carbon Co., Ltd., trade name
ATffi5 (graphite) Surface area 8 d/l...manufactured by Columbia Carbon Co., Ltd., product name RAVEN MT-P Surface area 30 d/l...manufactured by Tokai Carbon Co., Ltd., product name 5E
AST-5 Surface area, 45 d/l...manufactured by Columbia Carbon Co., Ltd.
Product name RAVEN 14 Surface area 85dl? ...Manufactured by Tokai Carbon Co., Ltd., product name 5E
AST-3 Surface area I40m"/f...manufactured by Cabot, product name V
ULCAN 9 surface area 1400dl? ...manufactured by Kabonoto Co., Ltd., product name BP-2000 R20% R in these current limiting elements (TP-20
), R7pg and TP measurement data, R(TP-2゜)/Rゎ, RTP/R21 calculated based on these data
The values of 1 are shown in Table 1. Incidentally, the island, R (TP-20) and R-PII were measured using a digital multimeter.

For comparison, Samples Nos. 8 to 16 were prepared in the same manner as Sample No. 1, except that the average particle diameter of carbon black, the surface area of graphite, and the amount of both were set as shown in Table 1. Data for the current limiting element is also shown.

(Effects of the Invention) As can be seen from the above embodiments, the present invention has an initial resistance value (R
ゎ) is not only low, but R(TP-2o)/difference is small and RTP/R, is large, and it has the characteristics of exhibiting remarkable PTC characteristics.

[Brief explanation of drawings]

1 to 4 are side views showing examples of the current limiting element according to the present invention.

Claims (2)

[Claims] (1) 40 parts by weight of conductive particles per 100 parts by weight of thermoplastic resin
~100 parts by weight are blended, and the conductive particles are carbon black with a surface area of 20 to 100 m^2/g and graphite with an average particle size of 10 μm or less, and the graphite has an abundance ratio in the total amount of conductive particles. 0.1～
0.6. (2) 40 parts by weight of conductive particles per 100 parts by weight of thermoplastic resin
~100 parts by weight is blended, and the conductive particles are carbon black with a surface area of 20 to 100 m^2/g and graphite with an average particle size of 10 μm or less, and the graphite has an abundance ratio in the total amount of conductive particles. is 0.1~
1. A current limiting element, characterized in that a conductive resin composition having a conductivity of 0.6 is molded into a predetermined shape, and two or more electrodes separated from each other are provided on the surface of the molded product.